Cardiovascular system abnormalities are nearly always apparent in sickle cell anemia. Increased cardiac output is necessary to compensate for reduced oxygen content of arterial blood. This is the consequence of anemia and of the lungs' inability to load the deficient remaining hemoglobin with oxygen. The circulatory system is strained by the sickled erythrocyte's propensity to occlude small blood vessels. Pulmonary vascular bed obliteration produces the most obvious effects. Abnormal ventricular performance is probably present but has not yet been adequately documented. Evidence for a specific myocardial lesion attributable to sickle cell anemia is tenuous. These mechanisms yield clinical findings associated with a high output state. In some, the pulmonary vascular bed obliteration is sufficiently advanced to produce clinical features of cor pulmonale. Heterozygous sickle states have few or no cardiovascular manifestations. This is true since anemia is less severe and vaso-occlusive events are less common in these disorders.